In recent years, in a cellular mobile communication system, with the advance of multimedia of information, the transmission of not only audio data, but also large-capacity data such as still image data or moving image data is being generalized. In order to realize the transmission of the large-capacity data, a technique for realizing a high transmission rate by using a high-frequency radio band has been being actively studied. However, when the high-frequency radio band is used, the transmission rate can be expected at a short distance whereas attenuation caused by a transmission distance is increased toward a longer distance. Hence, when a mobile communication system using the high-frequency radio band is really operated, a coverage area of a wireless communication base station device (hereinafter referred to “base station”) is reduced, as a result of which there is required that a larger number of base stations are installed. Because the installation of the base station is reasonably expensive, a technique for realizing the communication service using the high-frequency radio band while suppressing an increase in the number of base stations has been strongly demanded.
To meet the above demand, in order to enlarge the coverage area of each base station, as illustrated in FIG. 13, a relay transmission technique has been studied in which a wireless communication relay station device 20 (hereinafter referred to as “relay station 20”) is installed between a base station 10 and a wireless communication mobile station device 2 (hereinafter referred to as “mobile station 2”), and communication (indicated by arrows C and D in FIG. 13) between the base station 10 and the mobile station 2 is conducted through the relay station 20. With the use of the relay technology, a terminal (mobile station) that cannot communicate directly with the base station 10 can also communicate with the base station 10 through the relay station 20.
[Description of TD Relay]
In a TD relay, in a down link (DL), transmission from the base station 10 to the relay station 20 and transmission from the relay station 20 to the mobile station 2 are divided in time domain. Also, in an up link (UL), transmission from the mobile station 2 to the relay station 20 and transmission from the relay station 20 to the base station 10 are divided in time domain. In this way, the communication of a backhaul between the base station and the relay station, and the communication of an access link between the relay station 20 and the mobile station 2 are divided on a time axis so that a transmission time and a reception time of the relay station 20 can be divided. Accordingly, the relay station 20 can perform relay without being affected by loop-back between a transmission antenna and a reception antenna.
[Use of MBSFN Subframe]
In an LTE-A, in order to keep a mutual compatibility with the LTE, setting an MBSFN subframe at the time of transmission from the base station to the relay station in the DL has been studied. However, since the LTE mobile station has no function of receiving MBMS data, the mobile station receives only a control signal part, and ignores a data part other than the control signal part, in the MBSFN subframe. There is a possibility that the MBMS data is transmitted from a plurality of base stations. For that reason, there is a possibility that the LTE mobile station cannot accurately measure a reception quality of the base station connected with the subject mobile station. Accordingly, the LTE mobile station does not measure the reception quality in the MBSFN subframe.
Under the circumstances, the LTE mobile station sets a subframe that communicates with the base station as the MBSFN subframe in a cell of the relay station by using a feature that the mobile station does not receive the data part of the MBSFN subframe. With this configuration, the mobile station connected to the relay station can be prevented from measuring the reception quality in spite of no transmission in the subframe stopping the transmission because the relay station receives a signal from the base station.
As the MBSFN subframe used for the backhaul, there is used a subframe for really transmitting the MBMS data, or the MBSFN subframe other than subframes used for other purposes such as a CoMP or a positioning support.
Subsequently, a description will be given of a setting example of the MBSFN subframe with reference to FIG. 14. In FIG. 14, a mobile station 1 is a mobile station connected to the base station 10, and the mobile station 2 is a mobile station connected to the relay station 20. The relay station 20 sets a subframe #1 and a subframe #8 to an MBSFN subframe, and uses those subframes for transmission from the base station 10 to the relay station 20.
As illustrated in FIG. 14, the mobile station 1 can receive signals from the base station 10 by all of the subframes. Also, the mobile station 2 receives data from the relay station 20 by a subframe #0, a subframe #2, a subframe #3, a subframe #4, a subframe #5, a subframe #6, a subframe #7, and a subframe #9 other than a subframe #1 and a subframe #8 that are the MBSFN subframes. Further, the relay station 20 receives data from the base station 10 in the subframe #1 and the subframe #8 set to the MBSFN, and transmits data to the mobile station 2 by the other subframes.
Accordingly, a ratio of the number of subframes (the subframe #1 and the subframe #8) in backhaul to the number of subframes (subframes other than the subframe #1 and the subframe #8) in an access link of the relay station 20 is 2:8.